Lesson–4 : Laws of Motion

Stage–3 (Part–III) : Advanced Reasoning, Case-Based & Application Problems

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🔹 PART–K : ASSERTION & REASON QUESTIONS

1. Assertion (A): A body can move with constant velocity even if no force acts on it.

Reason (R): Force is required only to change the state of motion.

Answer:

Both Assertion and Reason are true and Reason correctly explains Assertion.

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2. Assertion (A): Action and reaction forces cancel each other.

Reason (R): Action and reaction act on different bodies.

Answer:

Assertion is false, but Reason is true.

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3. Assertion (A): A heavier body has more inertia than a lighter body.

Reason (R): Inertia depends on mass of the body.

Answer:

Both Assertion and Reason are true and Reason correctly explains Assertion.

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🔹 PART–L : CASE-BASED QUESTIONS (VERY IMPORTANT)

Case–1:

A lift of mass 800 kg is moving upward with acceleration. A person of mass 60 kg stands inside the lift.

(a) Why does the person feel heavier?

Due to upward acceleration, the normal reaction on the person increases.

(b) Write expression for apparent weight.

Apparent weight = N = m(g + a)

(c) What happens when lift moves with constant velocity?

Apparent weight becomes equal to true weight (mg).

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Case–2:

A player catches a fast moving cricket ball by pulling his hands backward.

(a) Which physical quantity is reduced?

Force of impact is reduced.

(b) Which law of motion is involved?

Newton’s Second Law (Impulse–Momentum).

(c) What happens to time of contact?

Time of contact increases.

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🔹 PART–M : MIXED NUMERICAL PROBLEMS (5–6 MARK LEVEL)

1. A force of 40 N acts on a body of mass 8 kg for 2 s. Find (a) acceleration and (b) change in momentum.

Solution:

(a) a = F/m = 40/8 = 5 m/s²

(b) Change in momentum = F × t = 40 × 2 = 80 kg m s⁻¹

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2. A man of mass 65 kg stands in a lift moving downward with acceleration 2 m/s². Find his apparent weight. (g = 10 m/s²)

Solution:

N = m(g − a) = 65(10 − 2) = 520 N

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3. A body initially at rest attains a velocity of 20 m/s in 4 s. If mass is 5 kg, find the force applied.

Solution:

a = (v − u)/t = 20/4 = 5 m/s²
F = ma = 5 × 5 = 25 N

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🔹 PART–N : HOTS QUESTIONS (HIGH ORDER THINKING)

1. Why is it difficult to push a loaded truck compared to an empty truck?

A loaded truck has more mass and hence greater inertia, requiring more force to change its state of motion.

2. Why does a swimmer push water backward?

According to Newton’s Third Law, water exerts an equal and opposite force on the swimmer, pushing him forward.

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🔹 PART–O : HOW EXAMINER AWARDS MARKS

• Correct law statement – 1 mark
• Correct formula – 1 mark
• Substitution – 1–2 marks
• Final answer with unit – 1 mark

Neat steps = full marks even if arithmetic mistake occurs.

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🔹 PART–P : QUICK REVISION POINTS

• Mass measures inertia
• Force causes acceleration
• Impulse = change in momentum
• Action and reaction act on different bodies
• Apparent weight changes in lift motion

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Stage–3 (Part–III) Completed ✔
Next: Stage–4 (IIT JEE Concepts & Advanced Applications)